1. Field of the Invention
In general, the present invention relates to a method for removing photoresist from locations on a semiconductor structure where its presence is undesired. One embodiment pertains to a method of removing residual photoresist (descumming) from a surface underlying the photoresist layer after patterning (opening) the photoresist. Another embodiment pertains to a method of removing a patterned photoresist layer from an underlying surface when the patterned photoresist is misaligned from its desired location on the underlying surface.
2. Brief Description of the Background Art
A patterned photoresist layer is often used as a template for pattern etching an underlying layer which functions as a hard mask during the etching of layers which underlie the hard mask. Frequently, the hard mask is formed from an inorganic material such as silicon oxide or silicon nitride (or a combination thereof).
FIG. 1 shows a typical prior art semiconductor structure 100 which includes, from bottom to top, a silicon substrate 102, overlaid with a layer 104 of an organic-based dielectric material, overlaid with a hard mask layer 106, and a patterned photoresist layer 108. After patterning of the photoresist layer 108, there are often xe2x80x9cfeetxe2x80x9d 110 remaining at the interface between the patterned photoresist layer 108 and the hard mask layer 106, as shown in FIG. 1. The presence of these xe2x80x9cfeetxe2x80x9d makes it difficult to maintain control of the critical dimensions of a pattern etched in the hard mask layer 106 (and the critical dimensions of features etched in the organic dielectric layer 104 underlying hard mask layer 106).
FIG. 2 shows a second prior art semiconductor structure 200, which may occur as an intermediate structure in a dual damascene process. This structure 200 includes, from bottom to top, a substrate layer 202, a contact via 210 etched in an organic dielectric layer 204, a patterned hard mask layer 206, and a second, patterned photoresist layer photoresist 214, which was deposited after etching of contact via 210. Any residual material from the first patterned layer of photoresist (not shown) which was used to pattern hard mask 206 was removed after etching of contact via 210. Then, an additional layer 214 of photoresist was deposited and patterned in order to provide a template for pattern etching the hard mask layer 206 to provide trenches which connect between contact vias (for example, and not by way of limitation). However, as illustrated in FIG. 2, an error occurred during patterning of second photoresist layer 214, and the opening 216 in second patterned photoresist layer 214, which was intended to be centered over contact via 210 is misaligned. By this time, considerable expense has been incurred in the formation of the structure 200 shown in FIG. 2, and rather than discard the structure 200, it is desired to remove the misaligned second patterned photoresist layer 214 and to reapply a new photoresist layer.
In the past, since most photoresists are comprised of organic materials, a plasma formed from oxygen (O2) gas has been used to remove the xe2x80x9cfeetxe2x80x9d 110 of the kind shown in FIG. 1 (descumming) and to strip misaligned photoresist from a semiconductor structure of the kind shown in FIG. 2 (reworking). However, even when a bias voltage is applied to the substrate, etching with O2 is isotropic. With respect to the structure illustrated in FIG. 1, this may cause a change in the critical dimension (an enlargement) of the pattern in patterned photoresist layer 108 by the time the feet 110 are removed. With respect to the structure illustrated in FIG. 2, this may cause undercutting of the organic dielectric layer 204 toward its base, as illustrated by the broken lines indicating areas 218. Also, etching with O2 is very fast, making it difficult to maintain good control over the amount of photoresist which is removed (or organic dielectric layer which is inadvertently removed).
It would be desirable to provide an effective process for descumming that would not result in a change in the critical dimension of the patterned photoresist. It would also be desirable to provide a method for removal of misaligned patterned photoresist that would not disturb an underlying organic dielectric material.
We have discovered a method of residual photoresist descumming which does not impact the critical photoresist pattern dimensions. In addition, basically the same etch chemistry can be used to strip misaligned patterned photoresist layers during a rework process without causing significant harm to layers of organic dielectric material which are separated from the misaligned patterned resist layer by an inorganic hard mask layer, or by an organic hard mask layer which is resistant to the etch chemistry of the method. These methods utilize a plasma etch where the plasma is generated from a source gas which is primarily (at least 50%) comprised of NH3. In many instances, the plasma source gas is preferably solely NH3. A relatively high substrate bias voltage (typically about xe2x88x92100 V to about xe2x88x921,000 V) is applied; the result is vertically directed anisotropic etching, where ionized nitrogen and hydrogen species, for example, are more active at the base of an opening in the substrate structure than at the lateral sidewalls of the opening. This avoids undercutting of the patterned photoresist layer during descumming. In the case of reworking, the hard mask layer shields the underlying organic dielectric layer at the top of the opening, and this, combined with the anisotropic direction of etch, reduces damage to the previously etched dielectric layer underlying the hard mask layer.